High-capacitance, low-inductance electrode for a short-wave therapeutic device

ABSTRACT

A short-wave therapeutic device is shown for the frequency range of about 10 MHz. to 100 MHz. (3 to 30m. wavelength) which has an inductivity and a capacity of such size that it is adequately in resonance with the frequency used. The device is particularly characterized by the use of a framelike-shaped armature made of a wide strip to provide inductivity which is as small as possible while making the capacity as large as possible by closely superposing the end portions of the armature.

United States Patent 2,223,447 12/1940 Hathaway Werner Haas Erlangen, Germany July 14, 1969 Jan. 11, 1972 Siemens Aktiengesellschaft Erlangen, Germany July 13, 1968 Germany Inventor App]. No. Filed Patented Assignee Priority HIGH-CAPACITANCE, LOW-INDUCTANCE ELECTRODE FOR A SHORT-WAVE THERAPEUTIC DEVICE 5 Claims, 10 Drawing Figs.

US. Cl 128/404 Int. Cl A6ln1/40 Field of Search 128/404,

References Cited UNITED STATES PATENTS 2,286,110 6/1962 Running 128/404 2,347,915 5/1944 Landaver 128/405 2,509,417 5/1950 Bowers..... 128/405 3,065,752 11/1962 Potzl 128/422 3,457,924 7/1969 Kendall et al 1. 128/404 FOREIGN PATENTS 954,128 4/1964 Great Britain..... 128/405 718,637 3/1942 Germany 128/413 1,110,773 7/1961 Germany 128/404 Primary Examiner-William E. Kamm Attorney-Richards & Geier ABSTRACT: A short-wave therapeutic device is shown for the frequency range of about 10 MHz. to 100 MHz. (3 to 30m. wavelength) which has an inductivity and a capacity of such size that it is adequately in resonance with the frequency used. The device is particularly characterized by the use of a framelike-shaped armature made ofa wide strip to provide inductivity which is as small as possible while making the capacity as large as possible by closely superposing the end portions of the armature.

PATENTEU m1 1 1972 3.1633588 SHEET 2 OF 5 MfRNE/Q HAAS IN EMToR ales-331588 PATENTED JAN] 1 i972 SHEET Q [1F 5 WERNER HAA5 IN ENTOR By AM A'rr QRMFYE HIGH-CAPACITANCE, LOW-INDUCTANCE ELECTRODE FOR A SHORT-WAVE THERAPEUTIC DEVICE This invention relates to a device for producing short wave therapy such as is required, for example, in conventional diathermy applications, the removal of adipose tissue, and other applications which are well known in the art.

In the conventional device utilized for this purpose there is generally provided a short wave radiation generator which has a frequency of about to 100 megacycles and a wavelength of about 3 to 30 m., a treatment electrode, and means for coupling the generator to the treatment electrode. Treatment electrodes have been generally of two types: the condenser electrodes and the loading coil section electrodes. Condenser electrodes are utilized in connection with the so-called condenser field method in which the person to be treated is placed in the high-frequency electrical field between insulated electrodes and thus acts as a dissipative dielectric. When the loading coil method is used, the person to be treated is placed in the high-frequency magnetic field ofa coil.

Conventionally, when either of these methods is used, the open circuit impedance of either the condenser electrode or the loading coil electrode should not deviate too much from one another if both types of electrodes are to be utilized in connection with the same generator, since otherwise severe damage would occur to the generator. conventionally, however, it was quite easy to tune loading coil section electrodes to resonance.

Therefore, conventionally, the art with respect to loading coil section electrodes has not developed during the past years. It has been conventionally believed that the short wave generators would have to be equipped with tuning devices so that, upon change of the load which was represented by the body disposed in operative relationship with treatment electrodes, the energy transferred to the body would be transferred optimally and any slow frequency drift of the generator would again have to be equalized. The conventionally automatic tuning device which has been developed for this purpose is expensive, noisy, trouble prone and, furthermore, a discrete time period is required until the tuning to resonance is accomplished. Because of the continuous drift around the point of resonance, in any automatic tuning device of this type, a uniform transfer of energy to the body under treatment will not be accomplished, which can cause problems such as insufficient heating or burning.

The present loading coil section electrodes suffer from a further disadvantage in that the fields which are created by such electrodes will fade relatively rapidly so that a field of sufficient strength will be present only within the immediate proximity of the electrode. However, if the body under treatment is brought into the immediate proximity of the electrode, the electrical fields at this location are very large and heating of the adipose tissue is too great. In order to produce a depth effect which has been achieved in short wave lengths through the use of open cavity resonators, a structure of this invention is required and the conventional loading coil section electrodes will not be satisfactory. The use of cavity resonators for the purposes of invention is medically impossible because such cavity resonators would have to be of size to correspond to an integral multiple of half the operating wavelength, which, in the case of conventional treatment wavelengths, would be 1.50 to m. In addition, the production of highfrequency energy in such wavelengths is considerably more expensive and the transmission loss is considerably greater than the frequency wavelength of about 3 to 30 m. mentioned in this specification.

By means of apparatus of this invention, a device has been created for short wave therapy within the frequency range of about 10 megacycles and 100 megacycles. This device is free from the disadvantages of the preceding structures, is cheaper to manufacture and is suitable for many different types of treatments such as, e.g., treatment for larger or smaller sections of the human body. The heat produced by this invention is well controlled and a depth, if desired, can be easily produced.

The invention may be briefly described as constituting a treatment electrode, which is connected to a short wave generator. The electrode has an inductance and capacity of proper size so that, at the frequency used, it will be almost exactly in resonance so that the capacity will be as large as possible and the inductance as low as possible.

This statement is adequate for technical treatment, since when constructing the electrodes of this invention, large capacities can be easily produced because, since the inductance is low, the condensers carry a far lower voltage load. While the capacity of the electrode can not be infinitely increased because the plate distances can not be reduced to minimal size because of thermal expansion and danger of breakdown, nevertheless relatively large capacities can be produced. The dimensions of the electrodes can not be made too large and the magnitude of the high-frequency voltage used limits the increase of capacity. Also, the type of connection between the transmission line and the electrode must be carefully considered because losses will occur in this area as well. However, with the use of this invention, and with air condensers, capacities in the neighborhood of 50 to 1,000 pF can be produced.

As a matter of theory of this invention, and while the inventor does not wish to be bound thereby, it is believed that, when the capacitive part of the electrode is increased and the inductive part decreased, the current intensity in the electrode becomes far greater. Therefore the magnetic field created within the electrode for the treatment becomes much stronger and a great penetration in depth is achieved. Furthermore, the considerable increase in current intensity will not produce the large amount of power loss involved in conventional electrodes.

The electrode of this invention is made in the form ofa large area armature. The applicant has demonstrated that, in the proximity of the limits of an electrode ofsuch form, the strong magnetic field produced by high-current intensity is rather homogenous and therefore the field intensity decreases rather slows with growing distance from the electrode, therefore excellent depth penetration is achieved. The shaping of the electrode as a large area armature has the further effect that the magnetic lines of flux are generally perpendicular to the body surface to be treated so that the electrical lines of force will run tangentially to the surface of the body. For this reason, the electrode is only slightly detuned when it is brought near to a patient or the patient is removed from its vicinity. Therefore it is possible to provide a connection between the electrode and the connecting line to the generator in which the generator will change the power emitted, depending upon the distance between the electrode and the patient because of the transformation of the effective resistance in the undamped circuit but the frequency will remain essentially constant. Therefore the electrode made in accordance with this invention can be used without automatic tuning means operatively connected to the generator and short wave apparatus made in accordance with this invention can be manufactured at far lower cost. While these devices would normally not be suitable for use with condenser electrodes, the advantages of this invention makes the condenser field method obsolete.

Furthermore, if the end faces of such an electrode are placed in confronting relationship with one another, the electrodes can be made small, thereby increasing their applicability for various types of treatment.

The above constitutes a brief description of this invention and the principal objects and advantages thereof. Other objects and advantages of this invention will become apparent to the reader of this specification as the description proceeds.

The invention will now be described by reference to the accompanying drawings, which are made a part of this specification.

FIG. 1 is a perspective view of a short wave therapeutic device made in accordance with this invention;

FIG. 2 is a top perspective view of a treatment electrode made in accordance with this invention;

FIG. 3 is a front perspective view of an alternative type of treatment electrode made in accordance with this invention;

FIG. 4 is a fragmentary cross-sectional view of the treatment electrode shown in FIG. 3 taken along lines IV-IV of FIG. 3;

FIG. 5 is a fragmentary perspective view of the form of treatment electrode shown in FIG. 3 as seen from below;

FIG. 6 is an exploded perspective view of another form of treatment electrode according to this invention;

FIG. 7 is a fragmentary cross-sectional view taken along lines VII-VII of FIG. 6;

FIG. 8 is a fragmentary cross-sectional view, on an enlarged scale, taken along lines VIII-VIII of FIG. 6;

FIG. 9 is a perspective view of a long field radiating system electrode made in accordance with this invention;

FIG. 10 is a cross-sectional view taken along line X-X of FIG. 9. I

The invention will now be further described by reference to the accompanying drawings. In this connection, however, the reader is cautioned to note that the specific forms of this invention, as shown in the drawings herein, are for illustrative purposes and for purposes of example only. Various changes and modifications could obviously be made within the spirit and scope of this invention.

The device of this invention includes a generator within housing 1, a carrying arm 2 which is connected to electrode housing 3, and a power cable 4 for connection between the generator and the electrode.

The electrode, in its simplest form, is shown in FIG. 2 and consists of a frame strip member which represents an armature. The ends 5 of this frame member are designed to overlap one another so that their surfaces run parallel to one another for some distance. Thus the electrode has a large capacity while the relatively wide strip has'only a slight inductance. A dielectric 6 can be disposed between the overlapping surfaces. The connection of the high-frequency feedline can be made to the electrode in any known manner. The strip used is polished, in order to obtain as smooth a surface as possible, and can be thin and flexible, so that the electrode-sheathed in flexible, insulating plastic material-can be fitted to the form of the parts of the body to be treated. The electrode shown has the following dimensions: width of frame b=200 mm.; length of frame l=450 mm.; width c or d, resp., ofconductor strip 40 45 mm.; material: aluminum, copper or silver-plated strips; dielectric: air (polyethylene, polystryene, or A1 0 ceramic). Thickness of strip: s e l mm. In the example shown, the width of the conductor strip is equal on all sides of the frame; it could also be selected differentially, it being particularly suitable, for obtaining a largely homogeneous magnetic field, to give them dimensions in inverse ratio as the lengths of the framesides, i.e., c1=bd.

In the electrode according to FIG. 3, the armature is formed by the frame parts 6a, 7, 8, 9 and 6b. At its open side, facing the viewer, the loop is adjusted to the form of the body by a concave arch. At the rear side, the electrode ends in an insulating plate 10. The connection of the coaxial feed cable 11 is arranged at the side of the frame 18. The outer conductor 12 is flanged to the side of the frame 8 by means of a metal angle 13. The inner conductor extends inside of the electrode and is connected there to the upward running strip 15, which then passes to the metal plate 16. This plate corresponds in its form and size to the metal plate 17 arranged opposite it, with which it is connected mechanically and conductively by means of the two metallic screw connections 18, 19. The metallic tongues 22 and 23 are attached by metal rivet 24 to the conductor part 6a, by way of the metal distance plates 20, 21. These tongues 22, 23 are so narrow that they fit well through the metal screw connections 18, 19 without touching same (see FIG. 3) and maintain a distance from them, so that no electrical flashovers will occur. To improve the stability over the entire length of the tongues, they are also connected by way of metal rivet 24a. 50 that the distance between the two tongues will remain Constant, the plate 25, which is made ofinsulating material, is

inserted between them. Not shown for reasons of clarity, but existing in reality are plastic strips which are arranged between the metal parts shown to maintain distance between them. Shown are only the distance rings 25a made of plastic material, which permit an intelligible drawing, in contrast to the plastic strips.

Welded on to the frame side 9 is the metal plate 26 extending in parallel to the frame part 6b. This plate 26 extends to the broken line 26a shown in FIG. 5, and its lateral limits are extended to tongues 26b and 260. The plate 26 and its tongues 26b, 26c, respectively, are mechanically connected to the parts 250, 25 and 612 by means of screw connections 27; in addition, these screw connections also form a conductive connection between 6b and 26 (26b, 260). The metal plate 28 has slots 28a and 28b, formed and arranged in such a manner that the bolts of the screw connections 27 serve as guide pieces engaging the slots for displacing the plate 28 in the direction of the double arrow 29. By tightening of the screw connection 27 the plate is stopped in the desired position and thus the magnitude of the electrode capacity is adapted to the requirements. The electrode is sheathed in a plastic housing as usual (not shown).

In this exemplified embodiment, the wide frame sides 6 through 9 form the small inductance, while the metal parts 6b and 26 or 28, respectively, form the one side, and the tongues 22, 23 connected to 6a form the other side of the large capacity. By means ofplates l6, 17 the inner conductor of the coaxial feedline is capacitively coupled to the tongues 22, 23 and thus to the electrode.

The particular characteristic according to the invention in this manner of manufacture of a large capacity is that practically no dielectric losses occur, because the only dielectric used (except for air) consists of the plate 25, which is situated in the (electrically) almost field-free space, because the condenser tongues 22, 23 have equal potential and the electrical lines of force between 22, 23 on the one hand, and 6b and 26, 28 on the other hand, do not run through this dielectric 25.

It can be seen from FIG. 6 that the electrode with the plastic housing 30 can be sheathed according to the vertical broken lines. The housing 30 has slots 33 on the sides 31 and 32 for the emission of heat. The housing is also provided with borings 34 and the electrodes have the corresponding threaded holes 35, so that the housing may be connected with the electrode by means of screws (not shown The electrode consists essentially of straight 79 angular metal plates which are arranged in a certain manner of space arrangement to one another by means of insulating plastic plates or plastic strips. The metal plates bear reference numbers 36 through 41; 45-47, 51, 58, 59, 64, 66 (FIG. 7), the insulating plastic plates are marked with reference numbers 67, 69, 72, 74 (FIG. 7) and the plastic strips, as distance pieces, bear the numbers 75-79 (FIG. 6), 79 (FIGS. 6, 7).

The square-shaped form of the electrode is provided by a repeatedly angular metal plate, whose individual sections are marked 36-41. At 42, 43, 44 there are welded to this metal plate the plates 45, 46, 47. The plate section 40 ends at the broken line 48 (FIG. 6) and the upper and lower limits of this plate are continued as tongues 49, 50. Along these tongues and movable in the direction of the double arrow 54 there is arranged the metal plate 51, which has slots 52 and 53. It can be fastened in the desired position by means of screws 55, 56. The plate 46 is bent at 57 and ends with the plate section 58. Plate 59 runs parallel to the plate section 58; it is bent at 60 and it is welded with its bent part to the inner conductor of the coaxial connection 62 shaped as a square bar 61; the outer conductor 63 of the coaxial connection is screwed to the metal plate section 36. Plate 45 continues in the plate section 64; in addition, the plate 45 is welded to plate 66 and 65. Thus, the plates 58, 59; the plates 39, 64, 66 and 46, as well as 45, 41, 47 and 40 and 51, respectively, run parallel to one another. The distance between the plates 41 and 47 is determined by plastic plate 67, whereby 41 and 47 are connected by means of a metal rivet 68 passing through plate 67. The

distance between plates 64 and 66 is determined by plastic plate 69, whereby 64 and 66 are connected by means of metal rivet 70 through plate 69. Plate 58 is connected to plastic plate 72 by means of rivet 71, and plate 59 is attached to plastic plate 74 by means of rivet 73. The plastic strips 75-79, 79' serve to maintain the distance, determining the distances, e.g., between the sections 45, 67, 49 (FIG. 8) and thus between the plates 40 and 49/50, respectively, and 47; 45 and 41; 39 and 64; 46 and 66; 72 and 74. The dimensions of the distances can be seen from the drawing, which shows an exemplified electrode on a scale of abt. 1:2. The plastic strips-just as the strip 79 visible in FIGS. 6 and 7-are also present at the bottom end ofthe electrode, which is not visible, and which is covered by the plastic plate 80; these strips are not shown for the sake of clarity ofthe drawing.

ln P10. 8 there is shown an enlarged section along lines VlIlVIll of FIG. 6, to promote better understanding. The screw 55 (56) serves to firmly attach to one another the sections 45, 76, 67, 75, 49, 51 (in a similar manner, sections 46, 78, 69, 77, 39 are attached to one another by means of screw 81 and nut 82), and in addition, the bolts of the screws 55 (56) constitute the guides for the tracks 52, 53.

The magnetic field created for the treatment exits at the opening covered with plastic plate 80, as well as at the side opposite to this opening, which is closed by the housing cover 83.

By means of the movable metal plate 51 the capacity of the electrode can be adjusted within certain limits and in this manner the electrode can be brought to resonance at a certain frequency.

FIG. 9 shows an electrode according to the invention, built out of two electrodes, with which long sections for treatment, e.g., the musculus erector trunci, can be heated.

This electrode consists of two armatures; these are formed oftwo repeatedly bent strips with sections 84, 85, 86, 87, 88 as well as 89, 90, 91, 92. The parts and 90 can also be replaced by a single piece of strip. The sections 88 and 94 are welded together at 93. The passing strip 97 is welded to the sections 87 and 92 at 95 and 96. The strip formed by sections 88 and 94 is connected to strip 97 by metal rivets 99, 100 with plastic plate 98 serving as separator between them.

Section 84 ends at the broken line 101; the lateral limits of this section 84 are extended into tongues 102, 103 to which are attached the threaded separator bolts 104, 105. These bolts serve as guide pieces for the slots 107, 108 provided in plate 106; in this manner, plate 106 is movable along the tongues 102, 103 in the direction of the double arrow 109 and can be fastened by means of knurled nuts 110, 111; it serves to adjust the capacity.

112 and 113 designate two metal plates which are conductively connected by means of screw connections 114, 115, with plastic strips 116, 117 as separators. The plastic strips are again present near the rear opening of the electrode, which is closed by means of plastic plate 118, and are designated there as 116' and 117. Additional plastic strips, which determine the distance between the sections 84, (89) and the strip 97, as well as the distance between strip 94 and metal plate 119 bear the reference numbers 120 and 121, and 120' and 121, respectively (FIG. 10).

The plates 1 19 and 84 (89) are connected to each other by means of the metal screw connections 122, 123 122, 123') and through inserting the plastic strips 120, 121 (or 120', 121 at the rear side ofthe electrode, respectively) as well as plastic plate 98 sufficient mechanical strength and secure maintenance of distance are assured. Plate 112 has a metal vane 124 to the free end 125 of which there is soldered the inner conductor 126, shaped as a square rod, of the coaxial connection 127. The outer conductor 128 is attached by means of screws 129 to the metal angle 130 mounted on the plastic rear wall 118, which metal angle in turn establishes, by means of screws 131, the electrical connection to the armatures, i.e., directly to the sections 84, 89. The plates 112, 113 serve for the capacitive coupling of the high frequency energy into the electrode. it must be noted that the large electrode capacity formed of the plates 97, 88 (94) and the plates 84 (89) and 119 contains air as dielectric which produces good stability and slight losses, and that the plastic plate 98 is situated outside the electrical field, thus is not causing any dielectric losses. No electrical fields of the capacity penetrate outside, either. The dimensions of this electrode are: length 1=80 cm.; depth t=abt. 10 cm.; width b=abt. 10 cm.; thickness of sheet abt. 1 mm., thickness of plastic plates and strips abt. l 3 mm., plastic material polystyrene or ceramic; sheet (and strip) aluminum.

As usual, the electrode is required, with vent holes).

As conductor material for the electrodes, metal sheet is used preferably, but within the framework of the invention it can be replaced by metallized plastic material, wire netting, or similar material. It is also within the framework of the invention to place several armatures, e.g., one inside another; in this or corresponding modifications, also according to the invention, higher inductance and greater electrode losses and lesser degree of effectiveness would have to be accepted.

To summarize, the electrodes according to the invention differ from those used today in shortwave therapy in that medically a greater depth effect of heating is achieved with a simultaneous, better fat removal. For this purpose, electromagnetic resonators are used, which have an opening, where there is an electromagnetic field that is advantageous for the treatment. The natural vibrations excited have in comparison with the known electrodes lesser voltages, small intensities of the electric fields, high currents, and large intensities of the magnetic fields, while showing the same capacity on the body to be treated. The electrodes are low-resistance," i.e., the inductances of the circuits are small, the capacities large. So that no great losses occur through the high-current intensities, large current-conducting surfaces are used. Through the large magnetic field in the opening and preferably perpendicular to the body under treatment, and the minor electrical field there, good fat removal is obtained. A great depth of penetration of the heating is also achieved through the magnetic field intensity dropping only very slowly with increasing distance from the opening. The magnetic field of the electrodes used in short wave therapy drops very rapidly, because the current conductors, in comparison with the desired depth of penetration, have only small cross-sectional areas.

In the electrodes described here, the current is therefore conducted over wide areas in relation to the desired depth of penetration. It was determined in tests that the depth, in which there is still produced one-half of the amount of heat as in the immediate vicinity of the electrode, corresponds approximately to the width of the armature, measured across the loop. A further measure towards increasing the depth of penetration consists in that the area set up by the current path is large. From the point of view of circuit organization, such a reaction is achieved of the electrodes to the generator with a different connection to the electrodes of the of the bodies treated, that the secondary circuit and its frequency fine-tuning of the usual short wave devices becomes superfluous. This behavior is caused by an input reflection factor of the radiating system, which at variable load on the electrodes by the body under treatment changes its amount very greatly and its angle only slightly so that the allowable range of the Rieke diagram is not exceeded. This favorable course of the reflection factor is due to considerably more magnetic than electrical energy being stored in the interaction space in front of the opening level in the uncharged electrode. The favorable course of the reflection factor can be adjusted through the selection of the coupling element and the tuning.

The energy emitted by the generator can be fed to the electrodes over a coaxial cable or a two-wire circuit. The feeding of power over the coaxial cable has the advantage of lesser radiation and secure operation, while power feeding over a two-wire circuit permits the connection of the electrodes to existing devices, above all when the electrodes are detuned in he capacitive range.

provided with a plastic housing (if The coupling of the natural vibrations of the radiating system with the fields of the circuit can be effected over electrical or magnetic probes. At overcritical coupling of the electrodes to the circuit, the length of the circuit is about v/4+m//2 for electrical probes, about vl2+nvl2 for magnetic probes (nintegral); through suitable dimensioning of the probe and the tuning, the length of the circuit may be somewhat varied by these values. The opposite applies to subcritical coupling. If the electrode is coupled overcritically to the circuit, there will be emitted by the generator, given proper tuning, a maximum output for large volumes to be treated; the opposite takes place with critical or subcritical coupling. The preceding considerations have been effected under the condition that the generator is open circuit stable. In the case of a short circuit stable generator the reverse conditions apply for length of cable and coupling.

The efficiency of the radiating system, i.e., the ratio between the power output to the patient and the power consumed in the electrode is large, because in the interaction space in front of the opening level a relatively large part of the entire magnetic energy is being stored, in contrast to the known inductive electrodes, in which a great part of the magnetic energy is stored in the immediate vicinity of the conductors, due to the comparatively small conductor sections.

The large capacities for large quantities of stored energy required for the electrodes are executed as large air condensers and a static or dynamic shielding of the capacity is provided, i.e., the electrical fields cannot get outside of the electrode. There are used, plate condensers in a closed metal box or in a dynamically shielded metal box.

The foregoing sets forth the manner in which the objects of this invention are achieve.

1 claim:

1. In a short wave therapeutic device, an electrode having the shape of at least one framelike armature consisting of a wide strip and having closely superposed end portions to provide large capacity and small inductivity.

2. An electrode in accordance with claim 1, wherein the armature is shaped to substantially completely enclose a hollow space.

3. An electrode in accordance with claim 2, comprising a plurality of armatures, said armatures being located mechanically one next to the other, and means interconnecting said ar matures electrically in parallel.

4. An electrode in accordance with claim 3, wherein parts of the armatures which touch each other are replaced by a part common to two armatures.

5. An electrode in accordance with claim 3, wherein each end of an armature consists of two plates equal in size and shape, whereby the plates of one end are enclosed by the plates of the other end and whereby the plates of both ends extend parallel to and opposite each other. 

1. In a short wave therapeutic device, an electrode having the shape of at least one framelike armature consisting of a wide strip and having closely superposed end portions to provide large capacity and small inductivity.
 2. An electrode in accordance with claim 1, wherein the armature is shaped to substantially completely enclose a hollow space.
 3. An electrode in accordance with claim 2, comprising a plurality of armatures, said armatures being located mechanically one next to the other, and means interconnecting said armatures electrically in parallel.
 4. An electrode in accordance with claim 3, wherein parts of the armatures which touch each other are replaced by a part common to two armatures.
 5. An electrode in accordance with claim 3, wherein each end of an armature consists of two plates equal in size and shape, whereby the plates of one end are enclosed by the plates of the other end and whereby the plates of both ends extend parallel to and opposite each other. 